The present invention relates to a method and an apparatus for manufacturing curved glass sheets. More precisely, the present invention relates to an efficient method and an apparatus for manufacturing curved glass sheets that are useful for example as window glass for buildings or vehicles.
Curved glass sheets are widely used for vehicles and buildings, and especially in the field of window glass for automobiles, there is a great demand for curved glass sheets due to design and aerodynamic considerations. Glass sheets that are mass-produced mainly using the float method primarily are formed into flat shapes. These flat glass sheets are formed into curved glass sheets in a secondary bending process, for which a number of industrial methods are known. Moreover, if the glass sheet is heated for the bending step, it is often quenched afterwards to temper it.
Methods for bending the glass that are widely employed include forming the heated glass sheet by sandwiching it with a pair of press molds. One type of these methods that is widely employed includes horizontal conveyance of the heated glass sheet to the press position with rollers for example, in order to avoid press marks from tongs on the surface of the glass sheet. As a means for horizontally conveying the glass sheet, rollers are common, but belts are also used.
For example, Publication of Unexamined Japanese Patent Application No. Hei 3-50132 (JP-A-3-50132) discloses a method wherein a glass sheet is carried out of a furnace, conveyed by a belt to a pressing position, and press-formed together with the belt. This method takes advantage of the belt""s flexibility to reduce the glass sheet""s temperature decrease between heating and quenching. That is, the glass sheet is bent while it contacts the belt, and it is also quenched in this situation. Furthermore, Publication of Unexamined Japanese Patent Application No. Hei 6-40732 (JP-A-6-40732) proposes methods for bending and tempering glass sheets that are improvements of this method.
However, regardless of the means for conveying the glass sheet, the glass sheet has to be temporarily stopped in the press mold during the forming step in these methods for bending glass sheets by press forming.
With regard to the manufacturing efficiency of continuous production of curved glass sheets, methods have been proposed wherein the glass sheet is bent without stopping it on the manufacturing line. In some of these methods, the glass sheet is heated and softened inside a furnace while conveying it horizontally, and using the glass sheet""s own weight, the glass sheet is gradually adapted to the surface form of for example an airbed for conveying the glass sheet. These methods (sag bending) are very efficient for continuous production of glass sheets having the same curved shape, and have been employed with various improvements (see for example Publication of Unexamined Japanese Patent Application No. Hei 7-237928; JP-A-7-237928).
Moreover, Publication of Unexamined Japanese Patent Application No. Sho 55-75930; JP-A-55-75930) discloses a method wherein a glass conveying passage having a curvature is set up as a continuation to the carry-out opening of the furnace, and the glass sheet is bent while being conveyed by rollers on this conveying passage. Compared to a sag bending method, this method has a better heating efficiency and the shape of the curved glass can be changed easier.
Other methods have been proposed, wherein, while conveying the glass sheet with rollers, the glass sheet is bent not only in the direction in which it is conveyed, but also in the direction that is perpendicular to the conveying direction (referred to as xe2x80x9ccross directionxe2x80x9d in the following).
For example, Publication of Unexamined Japanese Patent Application No. Hei 3-174334 (JP-A-3-174334) proposes a method, wherein rollers made of an elastic body are arranged above and below the glass sheet, and by applying a stress onto these roller pairs from the outside so as to bend them into a certain shape, the glass sheet also is bent in the cross direction Roller pairs for bending a glass sheet in a cross direction or for conveying a glass sheet that has been bent in the cross direction are disclosed for example in Publication of Unexamined Japanese Patent Application No. Sho 54-85217 (JP-A-54-85217) and No. Sho 55-75930 (JP-A-55-75930).
However, when bending the glass sheet with rollers that are arranged on both sides of the conveying passage, pressure is exerted locally on the surface of the glass. Consequently, there is the problem that roller marks often appear on the surface of the glass sheet. Scratches and bumps on the surface of the glass sheet caused by the rollers often lead to optical defects, especially in the field of vehicle window glass.
Furthermore, in continuous bending with rollers, there is the problem that the degree of freedom for forming the glass sheet and the precision are insufficient. When bending with rollers, the glass sheet is bent while it spans the rollers. Consequently, it is difficult to attain the desired bent shape at the front edge and the rear edge of the glass sheet with respect to the conveying direction. And if bending is performed with elastic deformation of the rollers, it is difficult to attain the desired bending shape precisely.
The present invention has been conceived upon consideration of these circumstances. It is an object of this invention to provide a method for bending a glass sheet while it is being conveyed, which has better manufacturing efficiency, where defects on the surface of the glass sheet do not occur easily, and where the degree of freedom and the precision for forming are improved. It is also an object of this invention to provide a manufacturing apparatus that is suitable for this manufacturing method.
In order to achieve these objects, a method for manufacturing a bent glass sheet in accordance with the present invention comprises heating a glass sheet in a heating furnace to a temperature where the glass sheet is changeable in shape, conveying the glass sheet out from the heating furnace, and bending the glass sheet by pressing the glass sheet against a bending member. The glass sheet is pressed together with at least one belt made of a heat-resistant material. The glass sheet is bent as the glass sheet is conveyed with the belt along the bending member. The bending member is curved at least in a direction that is perpendicular to a conveying direction of the glass sheet (i.e. the cross direction).
With this manufacturing method, the glass sheet can be continuously bent together with the belt. Therefore, the glass sheet can be manufactured with better efficiency, while defects on the surface of the glass sheet are suppressed. Moreover, the degree of freedom and the precision for forming are improved.
In order to achieve these objects, an apparatus for manufacturing a curved glass sheet in accordance with the present invention comprises a heating furnace for heating a glass sheet to a temperature where the glass sheet is changeable in shape, and a bending apparatus adjacent to the heating furnace so as to accept the glass sheet from the heating furnace and bend the glass sheet as conveying the glass sheet. The bending apparatus includes a conveying passage for the glass sheet, a bending member and at least one belt made of a heat-resistant material for conveying the glass sheet. The bending member is curved at least in a direction that is perpendicular to a conveying direction of the glass sheet (i.e. the cross direction). At least a portion of the belt is arranged along the bending member.
With this manufacturing apparatus, a curved glass sheet where defects on the surface of the glass sheet do not occur easily can be manufactured with better manufacturing efficiency, and with an improved degree of forming freedom and forming precision.